Solar power generation is becoming a progressively larger source of energy throughout the world. Solar power generation systems typically include one or more photovoltaic arrays (PV arrays) having multiple interconnected solar cells that convert solar energy into DC power through the photovoltaic effect. In order to interface the output of the PV arrays to a utility grid, a power converter system is used to change the DC current and DC voltage output of the PV array into a 60/50 Hz AC current waveform that feeds power to the utility grid.
Various power converter systems exist for interfacing the DC output of a PV array (or other DC power source) with the AC grid. One implementation of a power converter system includes two stages, a boost converter stage and an inverter stage. The boost converter stage controls the flow of DC power from the PV array to a DC bus or DC link (hereinafter referred to as the “DC link”). The inverter stage converts the power supplied to the DC link into a suitable AC waveform that can be output to the AC grid.
Many situations arise in which it is necessary to accommodate a PV array (or other DC power source) that has a high open-circuit voltage, such as an open-circuit voltage of about 1000 VDC or more. In such situations, it is desirable to have a power converter system that operates at a PV array source voltage and a DC link voltage that is less than the open-circuit voltage of the PV array. This is primarily because power electronic devices that are used in the power converter system, such as insulated gate bipolar transistors (IGBTs), are typically selected to accommodate the maximum power voltage of the PV array, not the open-circuit voltage of the PV array.
For instance, FIG. 1 illustrates a typical voltage-current curve 10 (hereinafter referred to as a “V-I curve”) for a PV array at a particular temperature and irradiance. FIG. 2 illustrates a typical power curve 20 for a PV array at a particular temperature and irradiance. The maximum power point for the PV array occurs at the current indicated by dashed line 12 in FIGS. 1 and 2. The voltage of the PV array at dashed line 12 is the maximum power voltage for the PV array. Point 14 of FIG. 1 represents the open-circuit voltage (voltage when the current zero) of the PV array. As illustrated, the maximum power voltage of the PV array is typically less than the open-circuit voltage of the PV array.
If the power converter system operates at a PV array source voltage or DC link voltage substantially equal to or greater than the open-circuit voltage of the PV array, the power converter system would require higher rated power electronic devices. Higher rated power electronic devices are typically more expensive and have higher conduction losses, leading to reduced efficiency. Moreover, the use of power electronic devices rated for a voltage higher than the maximum power voltage results in reduced operating efficiency. Thus, there is a need to have a power converter system that operates at a PV array source voltage and a DC link voltage that is less than the open-circuit voltage of the PV array.
Once a two-stage power converter system is running in steady state conditions, the inverter can regulate the DC link voltage such that the DC link voltage is less than the PV array open-circuit voltage. However, during startup or during other transient conditions when the PV array is first coupled, decoupled, or re-coupled to the converter, the power converter system may have to temporarily accommodate an open-circuit voltage or other high source voltage of the PV array. The transient conditions may cause the DC link voltage or the PV array source voltage to go above an over-voltage trip point for the power converter system, leading to damage or to disconnection of the PV array from the power converter system.
Thus, there is a need for a method and system to allow for a high source voltage in a power converter system during startup, shutdown, or other transient condition that maintains the DC link voltage and the PV array source voltage less than the open-circuit voltage of the PV array.